In recent years, an automobile equipped with the idling stop function has been widely diffused for the purpose of increasing gas mileage of the automobile. A lead-acid battery mounted on an idling stop vehicle is used in lower state of charge than a conventional engine-start lead-acid battery, and overcharge that invites generation of a large amount of gas is not performed. Therefore, the electrolyte solution is not fully agitated. As a result, the lead-acid battery for an idling stop vehicle tends to suffer the followings: the stratification of the electrolyte solution having occurred during charge-discharge cycles is not solved; irregularity in charge-discharge reactions in the longitudinal direction of the electrode plates becomes more pronounced; whereby a particular portion intensively deteriorates to reach the life of the battery (JP-A-2010-170939).
One scheme for suppressing stratification of the lead-acid battery for an idling stop vehicle is to increase the pore volume of the active material of the electrode plates. Thus, since the amount of electrolyte solution contained in the active material can be increased, the proportion of the charge-discharge reactions occurring in the electrode plates is increased. As a result, the progress of the stratification can be delayed. However, when the pore volume is simply increased, the density of the active material is reduced. This accelerates softening of the active material at the positive electrode plate, and a reduction in conductivity is invited at the negative electrode plate because of weakening in the coupling force in the active material. Thus, reactions occur irregularly, whereby stratification is accelerated.